The present invention relates to vapor coating and more particularly concerns apparatus for depositing thin coatings on a number of substrates. The invention is particularly useful for substrates having irregular surfaces.
Thin, uniform coating of various types of substrates is carried out for a number of purposes and in many different fields. Perhaps the most rapidly growing field in which such coating is performed is that relating to electronic equipment. The broad variety and great quantity of electronic devices being manufactured, sold and used throughout the world is largely a result of decreasing costs per electronic function which in turn is made possible by the extensive use of integrated circuits. Although small size and weight, together with increased reliability, have contributed to the demand and application of integrated circuits, it is the recent startling reduction in cost of fabrication that is primarily responsible for the rapid growth of this field.
Low cost is based primarily upon improvements in batch fabrications. Integrated circuits are made by simultaneously processing many wafers or substrates of materials such as silicon and silicon oxide. Each such wafer may bear hundreds of integrated circuits and thousands of transistors. Each is processed through a number of consecutive chemical and metallurgical operations. Critical dimensions are commonly measured in micrometers and sometimes in angstroms or tenths of a micrometer. Tolerances are small, commensurate with the critical dimensions themselves, and failure to meet such tolerances results in a defective device or a device of considerably shortened life.
A commonly employed type of integrated electronic circuit device is known as the metal-oxide-semiconductor device, also known as the MOS device. Details and some fabrication techniques of MOS devices are described in an article "Metal-Oxide-Semiconductor Technology" by William C. Hittinger in "Scientific American", August, 1973, pages 48 through 57. In MOS circuit fabrication, various types of silicon oxide, both doped and undoped, may be deposited upon a silicon base. Certain irregularities, often in the form of channels, are then formed on the surface of this multilayer substrate. The irregular or channeled surface is then uniformly covered with a thin coating of metal that is subsequently etched away in a predetermined pattern. The metal remaining provides electrical conductors along the surfaces of the substrate. Other steps, not pertinent to the present invention are employed, but not described herein.
The metal must be deposited in a coating of uniform thickness and density over the irregular surface, including bottom and both sides of the channels or other irregularities or depressions formed in the substrate. Because of the relatively sharp angulations of the surface formed by the various channels, difficulties have been experienced in providing coatings of a uniform thickness and density. The edges or brink of such channels are especially critical areas for such metal coating. Where metal coating has less than specified thickness, exceedingly high current densities may occur, current densities high enough in some cases to destroy the electrically conductive metallic lead and thereby destroy operation of the circuit.
In attempting to solve these problems of high quantity batch fabrication with uniform high quality and low rejection rate, many different tooling configurations have been attempted. Examples of these configurations are shown in U.S. Pat. Nos. 2,847,325 to Riseman et al, 3,128,205 to Illsey, 3,322,655 to Garibotti and 3,594,227 to Oswald. These patents describe structures that provide various motions of groups of substrates through and about the material to be deposited upon the wafer surface. These prior art devices may coat large numbers of wafers or substrates but all suffer from production of large numbers of defective coatings, particularly where the coating is to be applied to the irregular or channeled substrate surfaces.
For proper and reliably uniform coating of surface irregularities that may be measured in thousandths or even ten thousandths of an inch, it is required that the surface to be coated is presented at a selected acute angle to a line extending from the surface to the source of coating material. Prior art devices fail to maintain such angle in efficient and successfully productive tooling.
The prior art methods and devices involve apparatus of such great complexity that breakdown is frequent, inoperable time is great, and efficiency is low. Further, uniformity of coating thickness, particularly over edges of channels and grooves, is poor, and percentages of unacceptable wafers are undesirably large.
To provide uniformity of coating thickness, applicant's assignee has previously employed wafer metal deposition tooling embodying a number of superposed vertically stacked rings, each supporting a circular row of wafers, inclined with respect to a source of metal vapor, and all driven about a common vertical axis. As the rings are driven about the common vertical axis, each wafer is caused to rotate about its own axis and, thus, both sides of variously oriented channels on its surface are properly coated. In this prior system of applicant's assignee, there is no vertical sweeping of the wafers through the cloud of metal vapor within the evacuated chamber by any one wafer since each travels basically in a single circular horizontal path. Thus, the wafers individually and collectively fail to sweep through sufficiently large areas of the metal vapor cloud. Further, because of the large number of separately mounted moving parts and the requirement that many of these be demountable or at least accessible for ready detachment and attachment of the wafers themselves, this mechanism is subject to frequent breakdowns, requiring inordinately long times for repair.
Accordingly it is an object of the present invention to provide uniform coating of substrates both reliably and efficiently, while avoiding or minimizing the above mentioned disadvantages.